Musical instrument



Dec. 15, 1942. J, A. KOEHL MUSICAL INSTRUMENT Filed Oct. 24, 1940 2Sheets-Sheet 1 1942- J. A. KOEHL MUSICAL INSTRUMEIiT Filed Oct. 24, 19402 SheetsSheet 2 IN VEN TOR Patented Dec. 15, 1942 UNITED STATES PATENTOFFICE MUSICAL INSTRUMENT James A. Koehl, Chicago, Ill., assignor toCentral Commercial Company, Chicago, 111., a corporation of Illinois 22Claims.

This invention relates to electrical musical instruments of the classemploying means for producing sets of electrical impulses of a pluralityof different frequencies representing different note positions in amusical scale and adapted to be selectively combined to produce a toneof any desired timbre.

An object of my invention is to provide electrical sources in coactionwith and controlled by continuous simple tone frequencies from wellknown pulsation alternators, whereby selected timbres obtained by thesynthesis of said simple tone frequencies can, as desired, be combinedwith complex waveforms of continuous wave oscillations supplied by saidelectrical sources.

Another object is to provide sets of frequency sources in whichfrequencies developed by sources of one set are controlled byfrequencies developed by sources of the other set and wherebyfrequencies from either set of sources can be used independently of andfor musical results not possible of attainment with frequenciesdeveloped by sources of the other set.

Another object is to provide means enabling phase changes to be hadbetween frequencies with resultant production of an effect like the voixceleste stop of an organ, i. e., a single, double or triple organ-stoptuned sharp or fiat, or both, to produce an acoustical tremolo.

A further object is to simplify the construction of such instrumentswhile contributing to the musical advantages thereof.

Another object is to provide means for supplying such instruments with agreater number of higher harmonics than has heretofore been possiblewith instruments employing rotary or vibratile pulsation, generators,whereby more complex colors are obtained.

Another object is to provide a system of complex waveform generatorswhich are readily applicable to electrical musical instruments now incommon use and which employ either rotary or vibratile alternators whoseharmonically related output frequencies are of simple waveform andadapted to be synthesized into selected timbres.

A still further object is to provide one or more high impedanceoscillating circuits in electrical coaction with a low impedancepulsation alternator of simple tone frequency whereby, in response todepression of an associated playing key, either a simple tone frequencyproduced by said alternator or one or more complex waveforms from theoscillating circuits, or both the complex waveforms and said simple tonefrequency can be translated into sound at the will of the player.

A still further object is to provide in association with high and lowimpedance load circuits,

resistive output paths from high and low impedance generatorsrespectively in which the impedances in said paths are proportionedrelative to each other so that the amplitudes of voltages delivered tosaid high and low impedance load circuits from said paths are equal.

Another object is to provide in association with an output circuithaving high and low impedance sides respectively connected with high andlow impedance frequency sources so that the output voltage from saidcircuit is proportional to the combined voltages of any frequencydelivered from said sources.

My invention will be more clearly understood from the specificationhereinafter following and by reference to the accompanying drawings, inwhich:

Figure 1 is a schematic illustration of a two manual instrument in whichmy invention is incorporated, and

Figure 2 is a schematic illustration of a slightly modified formthereof.

In carrying the invention into practice, I may employ any well knownorganization of constant speed generating devices for producingelectrical vibrations corresponding to different notes of a musicalscale through a range of more than one octave. A system well suited tomy general purpose is disclosed and fully described in Letters Patent.to Thaddeus Cahill, No. 1,107,251, Aug. 18, 1914, and can be used withthe multiple key contactors disclosed in the Cahill Patent No.

1,295,691, Feb. 25, 1919.

In Figure 1 of the accompanying drawings, I show four constant speed lowimpedance generators respectively designated C3, C4, G4 and C5, eachcomprising a tone wheel rotor 4 of magnetic material provided withequi'-distantly related peripheral high points 5 of precalculatedshapes, and a pick-up magnet 6 associated therewith and provided with acoil 511 having a grounded terminal I and an output terminal 8, thelatter having a plurality of high resistive, parallel output paths 9. Asit is well known that such tone wheel rotors driven at assigned constantangular velocities and having predetermined numbers of peripheral highpoints, will induce periodic flux changes in the coils of theirassociated permanent magnets at rates corresponding to the tonefrequencies of different notes of a musical scale, it will sufi'icemerely to say that the four low impedance generators herein shown aredesigned, shaped, constructed and adapted to be driven at respectiveconstant angular velocities to produce l29.+, 258.+, 387.+, and 517.+,vibrations per second, respectively corresponding to the vibrationfrequencies of notes C3, C4, G4 and C5 of the even tempered musicalscale.

Figure 1 schematically represents an instrument having two keyboards,for each of which is a set or system of conductors or bus-bars H]. Ishow but a single playing-key for each of said key-boards, namely, thekey C3, and as illus trated, each key has a plurality of simultaneouslyactuable circuit controlling conta'ctors H, such that there is one suchcontactor in circuit controlling relation to each individual bus-bar ofa respective set of said bus-bars. As the two keys are associated withthe same note of the scale, it need only be said that each generatorsupplies frequencies to corresponding contactors ll of the respectivekeys.

For each keyboard of the instrument, I employ a harmonic control systemcomprising a plurality of multiple contact switches 12, each providedwith similar contacts [3 and an adjustable bridging member l4 therefor.Contacts occupying the same numerical order in each of the switchesconnect by a conductor IE to a preassigned tap Hi from a predeterminednumber of 3 turns of the primary or low impedance winding H of atransformer I511. One terminal of said primary winding connects with oneterminal of a potentiometer I8 whose variable contact member I!) for thehigh resistance No is connected by a common conductor 20 to the inputcircuit of an amplifier 2!. The other terminal of said potentiometerconnects with one terminal of the secondary or high impedance winding 22of said transformer, the opposite terminal of said secondary winding andone terminal of said primary winding being grounded at 23.

When a playing-key is depressed four circuits are simultaneouslycompleted, each'of which includes a coil 5a of the pick-up magnet of anassociated tone wheel rotor, and a resistance 9 which is sufficientlyhigh relative to the internal impedance of the generator of which saidwheel is a part and the remainin-g'portion of the circuit so assubstantially to determine the amount of current which will flow throughsaid circuit. Such circuits provide a low impedance load and are fullydisclosed and described in Letters Patent No. 1,956,350 of April 24,1934, to Laurens Hammond, and are employed in association with an outputcircuit which includes a transformer designed so that only a smallproportion of the electrical energy which might be generated is everutilized by a single circuit, even tho all of the circuits of the lowimpedance load should be completed at the same instant. I therefore makeno claim to this feature so far as it embodies use of such transformer,but point to it as an example of one well known means in which lowimpedance generators of simple tone frequencies may be satisfactorilyused in a low impedance load including means such as a transformerhaving a secondary winding whose number of turns is so in excess of thenumber of turns of the primary windings as to form part of a highimpedance load in which generators embodied in my invention can beconnected and their output frequencies rendered effective in said highimpedance load during transmission of content of the plate output.

simple waveforms of potential through the low impedance load circuits.

It is not intended that my invention si confined to use in connectionwith low iinpc ance generators of the electromagnetic type as s factoryresults can be had with circuits in which high impedance generators areemployed s disclosed in Letters Patent to Floyd A. Firestone, No.1,953,753 of April 3, 1934. Neither shall use be confined to anyparticular well known harmonic mixing system, nor to generators of sir.ple partial frequencies.

The upper keyboard of the instrument herein diagrammatically illustratedhas associated therewith a bus-bar 24 and as shown, the key C3 has inaddition to its contactors ii, a contactor 25 adapted to be closecircuited with said bus-bar when depressing the key to close circuitcon-- tactors II with their respective bus-bars it. Said bus-bar 24 isconnected by a conductor 22; to the high resistance Isa of saidpotentiometer l8 at its junction with the secondary winding 1? of thetransformer l5.

For each note in the instruments gamut, there is an oscillator 21preferably of the pentode vacuum tube type, having a controllingelectrode 28, screen grid 29, plate 36, suppressor grid 3! and filamentem. The tone wheel G5 has associated therewith a permanent magnet 32,having a coil 33, one terminal of which is grounded at 34 and the otherterminal connected to the control grid of said vacuum tube, whereby andsimultaneously with the production of flux changes in the coil of themagnet 6 like flux changes are induced in the coil 33 and impressed uponthe control grid of said tube so as to synchronize operation of saidtube with the simple tone frequency in the output of said magnet 6.

Voltage from a pure source of direct current of 150 volts under the loadimposed upon it, supplied the plate 39 through a resistor 35 ofapproximately 200,000 ohms in path 34. Current is supplied the screengrid through a stor 35 of approximately 50,000 ohms from path Thesuppressor grid is grounded through a, resistor 38 of approximately590,000 ohms and the cathode cm is grounded through a resistor 35?. Dueto the voltage drop across resistor 38, the control and suppressor gridsreceive a bias that is negative to the cathode. The resistor 39 thusequalizes differences in the tube characteristics by applying greaterbias if the tube tends to draw greater anode current, thereby reducingthe current to approximately normal and vice versa. The alternatingcurrent appearing across the resistor 33 degenerates the tube andreduces the harmonic A capacitor id of approximately .1 mfds. couplesthe screen grid to the suppressor grid whereby alternating potential isthe same at each of Since the control grid is supplied with a constantalternating current of the frequency determined by the tone wheel ofgenerator C3, the tube is predisposed to oscillate at some multiple orsub-multiple of the controlling frequency supplied the control grid asaforementioned. When the tube is tuned by capacitor ii, connected asshown, it oscillates at the same fundamental frequency as that ofalternating potential from said generator.

The tube has output paths 42 and from the plate and screen gridrespectively, each serving as a carrier of a waveform of alternatingpotential, the waveform from the plate having a smaller number ofharmonics than the number of harmonies taken oiT of the screen grid. Ineach of these output paths from the tube is a blocking condenser 44, ahigh resistor 45 and a stop. 46. These stops may be selectively close'circuited with the bus-bar 24, whereby either or both of said waveformsmay be conducted to the high impedance side of the transformer throughsaid conductor 26. At this point, it should be noted that theresistances 45 and the resistances 9 are of such relative values thatthe amplitude levels of voltages delivered to the high impedance loadare proportional to the amplitude levels of voltages delivered to thelow impedance load.

It is understood that each playing-key of the instrument has a contactoradapted to be close circuited with the bus bar 24 of the keyboard withwhich it is associated when depressing said key and that for each lowimpedance generator of simple tone frequency there is an oscillator 2'.whose fundamental frequency is the same as said simple tone frequency.In this manner, each key has at its command a simple waveform ofundulating potential and one or more complex waveforms of undulatingpotential. By connecting the simple waveform generators and theirinterconnected oscillators in an electrical network as shown anddescribed herein, any simple tone frequency can be translated into soundsingly or it may be combined with any selected number of other simpletone frequencies which are harmonically related thereto. combination ofsimple tone frequencies can readily be combined with one or more complexwaveforms delivered by said oscillators.

While I show but two output paths from each oscillator for each manual,additional paths can -1 readily be provided by use of capacitor systemswell known in the art, whereby many different waveforms can be produced.

The lower keyboard is of course connected in the system in the samemanner as the upper keyboard and it is thought that a description of onewill fully suflice for both. It accordingly follows that any quality canbe set up upon either or both keyboards by combining selected portionsof simple tone frequencies. It is further noted that one keyboard mayrender eifective upon the sound translating system selected complexwaveforms from the osci lators, whereas the other keyboard may rendereffective upon said system any desired number of simple tone frequenciesaccording to the quality of tone desired to be produced when playingsaid keyboard. I have not disclosed a pedal section as it is obviousthat same can be incorporated and its keys electrically connected withthe respective frequency sources in the same manner as already describedwhen referring to said upper and lower keyboards.

In Figure 2 two oscillation generators 50 and 5| are shown, such thatthe output leads 52 and 53 from the plate and screen grid respectivelyof generator 50 connect with stop switches 54 and 55, said switchesbeing normally open circuited relative to a common conductor 56. Thecontrol grid 51 of said generator may receive a controlling signal fromthe coil 33 of any magnet 32 shown in Figure 1. Assuming that the signalis received from coil 33 of magnet 32 associated with tone wheel rotorC3, the signal will have a frequency of 129.+ cycles per second,corresponding to the vibration frequency of note C3. If stop 54 is closecircuited with said conductor 55, a waveform of voltage will bedelivered to the input circuit of the amplifier 2| from the screen Inlike manner, any A close circuiting stop 55 a waveform of voltage isdelivered to said conductor 55 from the plate of said oscillationgenerator 50 and impressed upon the input circuit of said amplifier uponclose circuiting said key switch. When both stops are close circuited, awaveform of voltage which is the sum of the individual waveforms fromsaid plate and screen grid is impressed upon said input circuit of saidamplifier upon close circuiting said key switch.

It is understood that for each generator 50 employed in the instrumentthere will be one generator 5 I. This may be the same as generator 50 orit may be of any other well known type adapted to deliver a waveform ofoutput voltage, the frequency of which is either sharp or fiat relativeto the frequency of output voltage from generator 50. In the instantembodiment of my invention, the generator 5| has its output paths 59 and60 from the screen grid and the plate of the generator connected torespective switches BI and 52 which are each thereof connected with theaforementioned conductor 56.

In practice, upon close circuiting either or both stop switches 54 and55, either or both celeste switches 6| and 62 may be selectively closecircuited with said conductor 56 so that voltage from generator 5| whosefrequency is either slightly sharp or slightly flat relative to that ofthe output voltage from generator 58 will be combined therewith for theproduction of celeste effects. The control signal for said generator 51may be supplied from the coil of a tone wheel rotor of the instrumentthe output frequency of which is sharp or flat relative to the frequencyof output voltage from said generator 50, or said signal may be suppliedfrom any other well known source of controlling voltage.

I may use two oscillation beat note generators in association with agenerator of simple tone frequency instead of one as above described,one of which produces a frequency which is flat and the other afrequency which is sharp relative to said simple tone frequency.

Depending upon the number of harmonies desired to be produced by simpletone generators, I may use in the low impedance load any desired numberof generators, some furnishing higher or upper partial frequencies andothers sub-fundamental frequencies. In practice, good results are hadwith the system of generators disclosed in Letters Patent No. 2,187,910,to Valentine Mohler, dated Jan. 23, 1940, wherein twelve assemblies ofsimilar generators are employed, each driven at its own speed andprovided with tone wheel rotors, the number of high points on which areaccurately determined and shaped so that all the semi-tone frequenciesfor a scale in even temperament are produced. In this system, C3 note isproduced by a tone wheel rotor driven at an angular velocity of 981 R.P. M. and having sixteen peripheral high points. Note C4 is produced bya tone wheel rotor driven at the speed above stated, said rotor havingthirty-two peripheral high points. Note C5 is produced by a tone wheelrotor also driven at the speed above stated and provided with sixty-fourperipheral high points. The note G4 will be produced by a tone wheelrotor driven at 1470 R. P. M. and provided with thirty-two peripheralhigh point As the gist of my invention is in providin differentpluralities of generators of the same semi-tone frequencies, whichgenerators are in electrical coaction with each other so'that thefrequency generated by a generator of one of said pluralities ofgenerators is used both to provide a fundamental tone and to control thefundamental frequency of a complex waveform produced by a generator ofthe other plurality of generators, I am not to be limited to the formsof generators constituting either of said pluralities. Any well knowngenerator providing space current of complex waveform and capable ofbeing controlled and used as herein taught, may readily be substitutedfor the oscillators first above referred to.

I prefer to connect the keyboards to a single amplifying and soundtranslating system through the respective high resistances Isa. However,if desired, each of these resistances can be connected to its ownamplifying and translating system.

While I show separate pick-up magnets for the controlling electrodes ofthe aforementioned oscillators, controlling currents from the outputpaths of magnets E of said oscillators can be impressed upon saidcontrolling electrodes.

It also will be appreciated that while I show an oscillator for eachsemi-tone frequency throughout the instruments gamut, tubes employingmultiple controlling electrodes and emitting space currents of differentsemi-tone frequencies can be employed, thus materially reducing thetotal number of tubes necessary to be used. For example, an instrumentemploying ninety-six generators of simple tone frequencies will requirebut forty-eight tubes each having two controlling electrodes.

While I prefer to employ an oscillator, from separate electrodes ofwhich different waveforms may be separately or simultaneouslytransmitted to an electro-acoustic sound translating device, a A

space discharge device emitting but a single waveform but containing alarge number of harmonic components can be substituted therefor withoutdeparting from the spirit and intention of my invention and same can beconnected in any well known system having filtering means for modifyingsaid waveform and producing different timbres.

What I claim as my invention is:

1. In an electrical musical instrument, sources of relatively differentimpedance respectively producing simple and complex waveforms ofalternating potentials, means controlling the frequency of one of saidpotentials and including a constant speed alternator, means forutilizing the controlled potential to synchronize the frequency of theother potential therewith and including a transmitting path for saidcontrolling potential electrically interrelating said sources, and meansfor translating either of said potentials into sound.

2. In an electrical musical instrument employing rotary alternatingcurrent sources, separate sets of switches and connections between saidsources and said switches connecting switches of different sets to asingle one of said sources; a thermionic discharge tube for each of saidsources and including a controlling electrode connected therewith toreceive a controlling signal therefrom, and a system in coaction withsaid tubes, said switches and said sources for selective translation ofcurrent from the tubes and said sources into sound.

3. In an electrical musical instrument, a plurality of low impedancegenerators respectively generating variable currents at the tonefrequencies of different notes of a musical scale, an electrical networkhaving an output circuit provided with a transformer embodying a steppedlow impedance primary coil and a high impedance secondary coil, meansfor selectively conducting variable current from any generator to aselected number of turns of said primary coil, a variable currentgenerator of high impedance for generating a complex waveform of spacecurrent, said high impedance generator connected with a low impedancegenerator to receive a controlling signal therefrom to therebysynchronize the fundamental frequency of said space current with thefrequency of current from the low impedance generator, said highimpedance generator connected in the output circuit at the side of saidsecondary coil, and means for selectively conducting said space currentto said output circuit.

4. In an electrical musical instrument in which an alternator embodyingrelatively movable elements are formed and related to each other toproduce when in constant relative motion a sinusoidal waveform ofundulating potential of predetermined tone frequency; an electricalnetwork having a playing-key actuable therein to close circuit saidalternator with the input circuit of a vacuum tube amplifier and meansinterposed between said alternator and said input circuit for generatinga complex waveform of potential and impressing same upon said inputcircuit, and means electrically interconnecting said alternator and saidcomplex waveform generator for utilizing undulating potential producedby said alternator to control the frequency of the complex waveform ofpotential.

5. In a musical instrument employing an alternator for producingalternating potential at the tone frequency of a note of a musical scaleand a playing-key for controlling conduction of the produced potentialto the input circuit of a vacuum tube amplifier; a. thermionic dischargedevice having a controlling electrode, means including a circuit pathbetween said alternator and said controlling electrode for impressingcontrolling potential upon said electrode from said alternator, andselectively-actuable means connected for coaction with said key and.said thermionic discharge device to render space current developed bysaid tube effective upon said amplifier.

6. In a musical instrument, a translating device, a transmitting networkhaving separate paths over which alternating current potentials aretransmitted to said translating device, a key having contacts controlledthereby, a plurality of rotary pulsation generators having outputterminals respectively connected to said separate paths and to saidcontacts and with each other by a path over which frequency controllingalternating current potential from one generator is conducted to theother generator and means for selectively controlling delivery ofalternating current potentials from said generators to said separatepaths.

'7. In a celeste mechanism for electrical musical instruments,comprising an alternator producing output oscillations at apredetermined tone frequency, a vacuum tube generating outputoscillations at a beat frequency relative to the frequency of the outputoscillations from said alternator, and an electrical network in whichsaid alternator and said vacuum tube are connected so as selectively torender their output oscillations effective upon an electroacoustictranslating device.

8. In a musical instrument, high and low impedance loads respectively,generators of different simple tone frequencies including means forselectively close circuiting same with the low impedance load andcontrolling the relative amplitudes of said frequencies, oscillationgenerators, and means for selectively close circuiting said oscillationgenerators with said high impedance load.

9. In a musical instrument, high and low impedance loads respectively,low impedance generators of difierent fundamental frequencies, means forclose circuiting selected generators with said low'impedance load, highimpedance generators of complex waveforms and means for close circuitingselected high impedance generators with said high impedance load.

10. In an electrical musical instrument, a re tary generatorcontinuously generating a frequency corresponding to a tone ofpredetermined pitch, a generator continuously generating a waveform offrequencies having a tonal component corresponding to said predeterminedpitch, an output circuit, and a system in which said generators areconnected to enable their output frequencies to be selectively impressedupon said output circuit either conjointly or independently of eachother.

11. In an electrical musical instrument, the combination of an outputcircuit, a set of rotary generators of simple tone frequenciescorresponding to the vibration frequencies of the notes of the temperedmusical scale, a set of generators of complex waveforms of tonefrequencies, the first and second named generators electrically coupledtogether to control the operating frequencies of the generators of oneset of generators by the output frequencies of assigned generators ofthe other set of generators, and means including a keying mechanism anda system of stops in electrical coaction with the generators of bothsaid sets of generators and with the output circuit for selectivelyrendering said generators effective to produce in said output a waveformwhich is the sum of the output frequencies from any selected ones ofsaid generators.

12. In an electrical musical instrument, rotary generators continuouslygenerating predetermined tone frequencies; other generators continuouslygenerating tone frequencies, the waveforms of which are characteristicof tones of a higher harmonic content than those represented by thefrequencies produced by the first mentioned generators, said othergenerators coupled to the first mentioned generators to provide pairs ofgenerators in which the operating frequency of one generator of the pairis controlled by the operating frequency of the other generator of saidpair, an output circuit, and mechanism in electrical coaction with thefirst and second mentioned generators for selectively renderingeffective in said output circuit a waveform which is the synthesis oftone frequencies from any selected generators.

13. In an electrical musical instrument, the combination of an outputcircuit, a set of rotary generators of simple tone frequenciescorresponding to the vibration frequencies of the notes of the temperedmusical scale, a set of generators of complex waveforms of tonefrequencies, the first and second named generators electrically coupledtogether to control the operating frequencies of he generators of oneset of generators by the output frequencies of assigned generators ofthe other set of generators, and means including a keying mechanism inelectrical coaction with generators of the respective sets of generatorsand with said output circuit for producing in said output circuit awaveform which is the sum of the simple output tone frequency of agenerator of one of said sets of generators and the complex waveform ofoutput frequencies of a generator of the other set of generators whichis electrically coupled thereto.

14. In an electrical musical instrument employing tone wheel generatorsof simple tone frequencies, an electro acoustic translating device, andkeying means for controlling conduction of frequencies to saidelectro-acoustic translating device from selected tone wheel generators;a

thermionic discharge tube for each individual tone wheel generator andhaving a plurality ofoutput electrodes and a controlling electrode, acondenser-resistive circuit in which each individual thermionicdischarge tube is connected so that the'waveform of output frequencyfrom one output electrode of the tube is different from the waveform ofoutput frequency from the other output electrode of the same tuber, andmechanism embodying a system of stops electrically connected with saidtone wheel generators and said thermionic discharge tubes for renderingthe output frequencies. from said tubes selectively effective uponsaidtranslating device in response to actuation .of the aforementionedkeying means.

15. In an electrical musical instrument,the combination with anoscillatory circuit having means for continuously operating same toproduce therein a complex waveform of output potential; a continuouslyoperating pulsation generator connected by a circuit path with saidoscillator circuit for delivery to said circuit of a signal whichcontrols the frequency of operation of said circuit, a resistive outputpath coupled to -.said oscillatory circuit, and, .a resistive outputpath coupledqto saidpulsation generator.

16. In an -electrical,,musical instrument. employing a keyboard and anelectrical sound producer; oscillation generators of frequencies ofcomplex waveform, pulsation generators of waveforms corresponding tonotes associated with different playing keys of said keyboard, a pathconnecting each individual pulsation generator with one of saidoscillation generators for trans- .;mission to the latter from theformer of a signal which controls the frequency of operation of saidoscillation generator, an electrical network in which all of theaforementioned generators are electrically connected, means in saidnetwork for producing beat frequencies; and means in said networkresponsive to depression of any selected playing key of said keyboardfor simultaneously rendering eifective upon said sound producer acomplex waveform of frequency from an oscillation generator, a simplewaveform of frequency from an associated pulsation generator and a beatnote frequency from said beat note frequency producing means.

17. In an electrical musical instrument employing an electrical soundproducer; oscillation generators of frequencies of complex waveform,pulsation generators of waveforms corresponding to predetermined simpletone fre-.

quencies, a path connecting each individual pulsation generator with oneof said oscillation generators for transmission to the latter from andmeans in said network for rendering effective upon said sound producerwaveforms .ofhfrequencies from selected ones of any of said generators.

13. In an electrical musical instrument employing a keyboard and anelectrical sound producer; oscillation generators of I frequencies ofcomplex waveform, pulsation generators of Waveforms corresponding tonotes associated with different playing keys of said keyboard, a pathconnecting each individual pulsation generator with one of saidoscillation generators for transmission to the latter from the former ofa signal which controls the frequency of operation of said oscillationgenerator, an electrical network in which all of the aforementionedgenerators are electrically connected; and. means in. said networkresponsive to depression ofsany selected playing key of said keyboardfor. simultaneously rendering effective upon said. soundzproduceracomplex waveform .of frequenciesfrom an :oscillation generator andasimple waveformpf frequency from an associated pulsation generator.

19. In an electrical musical instrument, an output circuit, a lowimpedance load coupled. to said output circuit, a high impedance loadcoupled to said output circuit, a set of low impedance pulsationgenerators continuously producing frequencies corresponding .to' thevibration rates of predetermined musical tones and provided with highresistive output paths, a set of high impedance generators of complexwaveforms and provided with high resistive output paths, and respectivemechanisms for selectively impressing the outputs of the first andsecond mentioned generators upon said output-circuit through .theaforementioned high andilow impedance loads respectively.

29. In an electrical musical instrument, the combination with an outputclrcuit;;.a= set of low impedance alternating current generators, eachgenerator provided with a low impedance load;..means for 1 selectivelycoupling the. low impedance loads with said output circuit and forpredetermining the amplitude of alternating current delivered to saidoutput circuit from said low impedance generators; high impedancegenerators of alternating current, each high impedance generatorprovided with a high impedance load, and means for selectively couplingthe high impedance loads to said output circuit.

21. In a musical instrument, a thermionic discharge tube having acontrolling electrode and a path for the discharge of space current; analternator comprising a constantly driven element and a coacting pick-upelement, said alternator providing a source of variable current ofcontrolled, frequencypa circuit path interconnecting said source ofvariable current with the controllingwele'ctrodeof the aforementionedthermionic discharge tube for impressing said variable current upon saidelectrode so that space current generated by said tube has the samefundamental frequency as said variable current; andmeans by which eitherthe controlled variable current or said space current or both saidvariable current and the space current can be translated into sound.

22. In a musical instrument, a playing key; a source of variable currentof controlled frequency and comprising a generator embodying relativemovable coacting elements; a thermionic discharge tube having acontrolling electrode and a path for the discharge of space current,said path interconnecting said generator with said controlling electrodefor impressing said controlled variable current upon said controllingelectrode; and selectively actuable means responsive to depression ofsaid key for translating eitherthe space current or the controlledvariable current or both thereof into sound.

JAMES A. KOEHL.

